PROJECT SUMMARY Type 1 diabetes (T1D) is a life-long disease requiring daily injections of exogenous insulin. The incidence of T1D has been increasing worldwide in recent decades. Genetic susceptibility and environmental factors interactively contribute to T1D development. Human genetic studies have identified more than 50 loci significantly linked to T1D. However, our knowledge of the underlying genes within these regions that independently or cooperatively influence cellular processes leading to the destruction of insulin producing pancreatic beta-cells is incomplete. To fill this gap, we used nuclease based approaches to target the murine orthologs of human T1D candidate genes directly in nonobese diabetic (NOD) mice to seek functional evidence of their roles in diabetes development. In this effort, we discovered that Il27 is essential for diabetes development in NOD mice. Both CD4 and CD8 T cells are required for T1D development. How beta-cell autoreactive T cells are activated, accumulate, and maintain their effector function during T1D progression has not been fully defined. In this application we will determine the mechanisms by which IL-27 impacts T1D through its direct effects on T cells. IL-27 exerts diverse immunological functions by binding to its receptors (IL-27Ra and gp130 heterodimers) expressed on many immune cells, including macrophages, dendritic cells (DCs), B cells, and T cells. We showed here that both NOD.Il27-/- and NOD.Il27ra-/- mice, respectively lacking IL-27 and its receptor, were completely resistant to diabetes, indicating a critical role of IL-27 signaling in T1D development. Our studies also demonstrated that IL-27 produced by macrophages and/or DCs was sufficient to drive T1D but their responses to IL-27 were not important for diabetes development. Total T cells isolated from NOD and NOD.Il27-/- mice were similarly diabetogenic when transferred into IL-27-sufficient NOD.Rag1-/- recipients. Thus, ?-cell autoreactive T cells are present in NOD.Il27-/- mice, but their pathogenic activity cannot be induced or sustained in the absence of IL-27. In contrast, total T cells isolated from NOD.Il27ra-/- mice did not induce T1D in NOD.Rag1-/- recipients, indicating that direct IL-27 signaling in T cells promotes diabetes development. Using a mixed CD4 and CD8 T cell transfer approach, we further demonstrated that IL-27 signaling in both CD4 and CD8 T cells is important for T1D progression. One important diabetogenic activity of CD4 T cells is to provide help to autoreactive CD8 T cells that directly recognize and kill insulin-producing beta-cells. Therefore, we hypothesize that IL-27 is important for the accumulation and sustained effector function of beta-cell autoreactive CD8 T cells by directly acting on them and indirectly through enhancing the helper function of CD4 T cells. We propose the following aims to test this hypothesis. (1) To identify the mechanisms by which IL-27 intrinsically promotes beta-cell autoreactive CD8 T cells in T1D. (2) To determine the function of IL-27 signaling in CD4 T cells for supporting autoreactive CD8 T effectors in T1D. Completion of the proposed experiments will lay the framework for future human studies.